1. Field of the Invention
The present invention relates to a circuit for controlling an electronic ballast and, more specifically, to a phase detection circuit for controlling an electronic ballast.
2. Description of the Related Art
Controlling the brightness (.PHI.) given off by a fluorescent lamp being powered by an electronic ballast requires a circuit whose type can be classified as either "open-loop" or "closed loop". In an "open-loop" design, the controlling circuit knows nothing about what is occurring at the output. The operating point(s) are predetermined and fixed, regardless of changing conditions on the lamp or the ambient temperature. When a high quantity of ballasts which incorporate open-loop control are produced, trimming is required to account for component tolerances. Because the component tolerances alone can be large, the control circuit itself must have high accuracy which increases the cost for a good design. Furthermore, the circuit is still subject to lifetime effects of the lamp and changing ambient temperature.
In contrast, in a "closed-loop" design, information from the output is fed back to the control circuit allowing the circuit to automatically adjust itself for component tolerances, lamp life effects and temperature. Closed-loop control also allows for the lamp to be dimmed with extreme accuracy, which is especially important when a ceiling is filled with lamps, all of which should have the same brightness, particularly at low light levels, where differences from lamp to lamp are more readily detectable with the human eye. Closed-loop circuits also require less accurate designs, therefore reducing costs.
Referring to FIG. 1, one of the most common solutions to controlling the brightness of a fluorescent lamp using a closed-loop approach is to sense the lamp current with the use of a transformer 2. This allows for the lower cathode 4 of the lamp to be heated with the same current as the upper cathode 6, and allows for the lamp current to be separated from the heating current so it can be measured independently.
The lamp current can then be sensed with either a resistor or a second transformer 8. The secondary output of the transformer 8 is then rectified and low-pass filtered before compensated and summed with a reference voltage (REF). The resulting error (ERROR) then tells the control circuit to either increase or decrease the lamp current (usually by changing the frequency of a squarewave (VIN) driving a series/parallel RCL lamp resonant circuit consisting of inductor 12, capacitor 14 and the lamp) depending on whether the feedback signal (VFB) is higher or lower than the desired reference (REF).
The above-described classic control loop, however, has an inherent error due to the non-linear operation of rectification and has a high component count (2 transformers, rectifying diodes, compensation network, error amplifier, etc.).
Other solutions exist as well, but all require the use of a transformer of some sort to sense the actual lamp current.